1. Field of the Invention
The present invention relates to polymer coated metal powders, such as metal powders used in the formation of solder alloys, spheres and pastes. The metal powders are coated with a thermally decomposable polymer, such as a cyanoacrylate polymer or a dicyclopentadiene (“DCPD”) oligomer.
2. Brief Description of Related Technology
Solder is widely used in the assembly of semiconductor packages and semiconductor devices.
For instance, solder balls or spheres are used in the assembly of semiconductor packages, such as in flip chip applications.
Solder paste is commonly used for surface-mounted soldering of electrical components to circuit boards. Solder paste is useful because it can be applied to selected areas of the circuit board with its tackiness characteristic providing the capability of holding the electrical components in position without additional adhesives before forming the permanent bonds as the board passes through the solder reflow process.
Solder paste typically comprises a solder powder, a resinous component such as rosin, activators such as organic acids or amines, rheological control agents, thickeners and solvents. The solder paste is typically coated on the circuit board by techniques such as screen printing, dispensing, and transfer printing. Thereafter, the electrical components are placed on the circuit board and the solder paste is reflowed, by which the solder is heated sufficiently to cause it to melt and thereafter is cooling the solder sufficiently to cause it to solidify.
One problem in the industry associated with the use of solder paste is that it often has a short and unpredictable shelf life, e.g., typically from about one month to six months. The unpredictability in shelf life is caused, at least in part, by variations in the lag time from when the solder powder is made to the time it is mixed with flux to form solder paste, thereby resulting in variations in the degree of oxidation on the solder powder. Such oxidized powder does not reflow as well as unoxidized powder. Further, when the solder powder is combined with flux, which is inherently corrosive, the solder powder often reacts with the flux, thereby oxidizing the powder and reducing the acidity, thus effectiveness, of the flux. As a result, the performance of the solder paste often deteriorates over time. Moreover, the reaction between the solder powder and the flux typically causes the viscosity of the solder paste to increase substantially, which can make printing the solder paste difficult if not impossible depending on pitch.
Attempts have been made to reduce the reaction rate between the solder powder and the flux and thereby increase the shelf life of the solder paste, by storing the solder paste under refrigeration conditions. However, refrigeration is not effective to compensate for the varying degrees of oxidation on the solder powder prior to its incorporation into the solder paste.
It has also been reported that solder powder has been coated with materials that are non-reactive with the solder paste. For example, U.S. Pat. No. 4,994,326 discloses that coating agents that are insoluble or hardly soluble in a vehicle for solder pastes including those based on silicone and fluorine such as, for instance, silicone oils, silicone base high-molecular compounds, fluorinated silicone oils, fluorosilicone resins and fluorinated hydrocarbon base high-molecular compounds, are used as coatings.
The '326 patent also discloses a relatively large mount of coating material which is applied to the solder powder. While the relatively large amount of coating material may be effective to inhibit oxidation of the solder powder, in general, large amounts of coating material are undesirable since they can create a barrier which can inhibit the reflow of the solder. Moreover, such large amounts of coating material may cause physical obstructions and/or impurities which result in poor reflow characteristics, such as inadequate substrate wetting by the flux which can cause poor spreading of the solder and a discontinuous solder connection.
In addition, the '326 patent discloses the use of fluorinated hydrocarbons which are used as solvents in coating the solder powder. Currently, fluorinated hydrocarbons are considered to be an environmental pollutant and the use thereof is generally undesirable.
U.S. Pat. No. 6,416,863 is directed to and claims a method of encapsulating solder metal powder in which the powder is provided with a thin polymer protective layer by a polymerization reaction running on the surface of the solder powder, with the following steps:                a) producing a suspension of powder and a hydrophobic liquid,        b) generating a hydrophobic surface layer on each metal particle by adding a cationic tenside with a chain length of C1 through C20 with continuous stirring to form a brush structure on the hydrophobic layer of step (a),        c) stirring the mixture of steps a) and b) until formation of a viscous homogeneous mass,        d) adding a radically polymerizable monomer to the mass of step c) and which forms a thermoplastic polymer with a glass temperature Tg of at least 60° C. below the solidus temperature of the solder powder,        e) adding an organic initiator to start an interfacial polymerization reaction with incorporation of the hydrophobic layer of step b) and formation of a protective layer of thermoplastic polymer which has fluxing agent characteristics,        f) introducing the mass of step e) into an aqueous preparation with continuous stirring, whereby the preparation contains an emulsifier for suspension stabilization and controlling the polymerization reaction by tempering to 50° C. to 90° C. and maintaining this temperature for at least 120 min, and        g) cooling, washing and recovering the encapsulated solder powder of steps e) and f). Suitable monomers for forming the encapsulating wall are reported to be radically polymerizable monomers, preferably methacrylic-2-hydroxyethylester or methylmethacrylate.        
U.S. Pat. No. 5,328,522 is directed to and claims a solder paste comprising                (i) a flux and        (ii) a coated solder powder comprising solder particles coated with parylene (having a melting point lower than the solder particles) in an amount from about 0.001 to about 0.5 weight percent based on the total weight of the coated solder powder and effective to inhibit oxidation of the solder particles in the solder paste without substantially inhibiting reflow characteristics of the coated solder powder.        
U.S. Pat. No. 4,452,861 (Okamoto) describes solid particles encapsulated with cyanoacrylate polymer. The particles are encapsulated to prevent degradation due to reactive or corrosive surroundings. The cyanoacrylate polymer is used to coat phosphor particles and the like which are employed as coatings in cathode ray tubes and the like. Cerium activated calcium sulphide phosphor powder is the exemplified material which is coated.
U.S. Patent Application Publication No. 2005/0171273 describes a curable composition for forming anisotropically conductive bonds comprising:                (i) an amount of a first substantially uncured curable component; and        (ii) conductive particles coated with the cured product of a second curable component, where the coated-conductive particles are dispersed within the first curable component.        
Notwithstanding the state-of-the-technology, it would be desirable to provide a curable composition as a coating on metal particles particularly those suitable for use in solder pastes.